The present invention relates generally to a termination assembly for a power cable. More particularly, the present invention relates to a power cable termination assembly used for power cable testing after manufacture.
Quality testing is performed on a length of finished power cable at the end of the manufacturing process. In the art, one such quality test is a high potential test where the finished cable is subjected to an alternating current voltage of approximately three to four times the tested cable""s rated operating voltage. With this voltage applied, partial discharge, also referred to as corona, from the cable is measured to determine the insulation quality. If partial discharge greater than a specified magnitude is present under such high potential testing, the tested power cable is defective and probably contains voids or contaminants within the cable insulation. Basically defined, partial discharge is the phenomenon whereby air ionizes and begins to conduct electricity under high voltage conditions. In some cases, partial discharge can produce light, noise, and even ozone.
During the testing process, however, if the cable end is not properly terminated, partial discharge will occur at the cable termination even when the insulation contains no defects. The existence of partial discharge at the termination can hinder the detection of flaws in the cable insulation because when partial discharge occurs at the cable termination, its origin may be difficult to determine. In this case, testing techniques cannot distinguish the source, which may either be a defect in the cable insulation or ionized air at the cable termination. When a material with a high ionization voltage surrounds a cable termination, however, the ionization at that termination is greatly reduced or nearly eliminated.
FIG. 1 shows an exemplary conventional cable 100 containing a jacket 110, a primary shield 120, an insulation shield 130, insulation 140, conductor shield 150, stranded conductor 160, and strand seal 170. Those skilled in the art will appreciate that other cable constructions, including various voltage ratings, may be used in conjunction with the present invention.
Jacket 110 provides thermal, mechanical, and environmental protection of the layers underneath it. Jacket 110 is optional and may be constructed of polyethylene, PVC, or nylon.
Next to the jacket, primary shield 120 may be made of a circumferentially corrugated metal tape, drain wires or a concentric neutral. A concentric neutral 120xe2x80x2, shown in FIG. 2, comprises a plurality of electrically conductive strands placed concentrically around insulation shield 130. The concentric neutral 120xe2x80x2 serves as a neutral return current path and must be sized accordingly. The insulation shield 130 is usually made of an extruded semiconducting layer that is partially bonded to the insulation 140. Primary shield 120, insulation shield 130, and conductor shield 150 are used for electrical stress control providing for more symmetry of the dielectric fields within cable 100.
The insulation 140, contained beneath insulation shield 130, is an extruded layer which provides electrical insulation between conductor 160 and the closest electrical ground, thus preventing an electrical fault. Generally, insulation 140 is made of polyethylene, crosslinked polyethylene, or ethylene-propylene rubber. Polyethylene is susceptible to degradation due to partial discharge which may in turn lead to xe2x80x9cwater treeingxe2x80x9d. Water treeing is the phenomenon whereby small tree-like voids form and grow in the insulation 140 and fill with water that may have ingressed through the conductor strands. If a tree grows large enough in the insulation 140, electrical breakdown, and thus cable failure, will occur between the conductor 160 and an electrical ground. Crosslinked polyethylene is a significant improvement to polyethylene and, like ethylene-propylene rubber, is less susceptible to electrical breakdown due to water treeing. Also, ethylene-propylene rubber is more flexible than polyethylene or crosslinked polyethylene.
Conductor shield 150 is generally made of a semiconducting material and surrounds conductor 160. As stated previously, conductor shield 150 is used for electrical stress control, providing for more symmetry of the dielectric fields within the cable 100. Conductors are normally either solid or stranded, and are made of copper, aluminum or aluminum alloy. The purpose of stranding the conductor is to add flexibility to the cable construction. The small spaces between the strands of a stranded conductor, however, provide a path for water to ingress the cable 100. As stated previously, water can aggravate the treeing problem within insulation 140, accelerating cable failure. In an attempt to alleviate this problem, strand seal 170 is added into the small spaces between the strands. While the strand seal tends to limit the water ingress, it does, however, add to the stiffness of the cable 100. A solid conductor 160xe2x80x2 construction is shown with respect to FIG. 2, as an example of a construction not requiring strand seal.
FIG. 3 illustrates an exemplary conventional cable prepared for a termination assembly. To prepare the cable for termination, jacket 110 (not shown) and primary shield 120 (not shown) are first removed for a predetermined distance, exposing insulation shield 130. Second, insulation shield 130 is cut circumferentially at a predetermined distance from the end of the cable and that portion of insulation shield 130 is removed from the insulation 140. At last, a circumferential cut close to the end of the cable is made through insulation 140 and conductor shield 150. After this cut is made, a portion of the insulation 140 and conductor shield 150 at the end of the cable are removed, exposing conductor 160 at the cable end. When testing the quality of the insulation, cable manufacturers currently employ several techniques to limit partial discharge not caused by defects in the insulation.
For example, most cable manufacturers currently use resistive paint as a mechanism for limiting partial discharge at a cable termination. Using this method, the insulation shield is cut circumferentially at a certain distance from the cable end. Care must be taken to cut completely through the insulation shield without scoring the insulation. The insulation shield at the cable end is then removed, exposing the insulation surface. A thin layer of resistive paint is applied over the exposed insulation surface around the cable circumference overlapping the insulation shield by about one inch and extending to near the end of the cable. The layer of resistive paint provides a resistive current path, which results in a linear distribution of the electrical stress along the cable end, thus limiting partial discharge.
The effectiveness of this method, however, depends heavily on the cable construction, characteristics of the resistive paint, and the length of the insulation shield. Different cable insulating materials or different cable sizes, therefore, require resistive paint of different electrical characteristics or different insulation-shield-strip-back distances. This requires the technician performing the test to have a variety of different resistive paints available and to apply a different strip-back distance for the variety of different cables tested, which adds to the complexity of the testing process.
Another major problem associated with the resistive paint method is that it requires special handling because the paint dries quickly and chips very easily. Once the paint chips, the chips can act as sharp electrodes and can cause significant partial discharge in the area, which aggravates the very problem the resistive paint was intended to overcome. In addition, the resistive paint is not completely compatible with ethylene-propylene rubber, a commonly used cable insulation. Due to the chemical characteristics of ethylene-propylene rubber, when resistive paint is applied, bare spots may form on the painted area of the insulation, resulting in partial discharge when a voltage is applied to the cable.
The stress-cone method is less frequently practiced by cable manufacturers. A stress cone is a prefabricated cable termination device made of a high permittivity material. Once applied to a cable of an appropriate design, a stress cone provides stress relief. To install the stress cone, the cable end is prepared in the same way as with the resistive paint. In preparing the cable end, the insulation shield is cut circumferentially at a certain distance from the cable end. Care must be taken to cut completely through the insulation shield without scoring the insulation. The insulation shield at the cable end is then removed, exposing the insulation surface. After the cable is prepared, the stress cone is slipped over the cable and sits over the insulation shield cutback. When installing the stress cone, silicon grease is normally used as a lubricant and as a filler for the voids that may be present between the cable and the stress cone. The drawbacks of stress cones are: 1) they are available in limited voltage ranges; 2) they must be kept in inventory, thus increasing inventory costs, and 3) they have poor range-taking capability. Each drawback tends to increase the testing process cost.
A third and rare practice is to use stationary oil bottles made of Plexiglas(copyright). These bottles are about two feet long, six inches in diameter and are mounted on stationary stands. Each bottle is filled with hardened steel balls (such as ball bearings) and has a bottom cap made of aluminum. A brass electrode is connected to the outside of the bottom cap. To terminate the cable, the various cable layers are stripped back to various lengths. The exposed conductor end is placed between the steel balls in the bottle filled with dielectric oil.
The majority of cable manufacturers have abandoned this practice because of its many drawbacks. These drawbacks include: 1) excessively long cable ends increase scrap costs; 2) the stands necessary to support the bottles take up a large area and significantly reduce the number of cables that can be tested at one time in a given area; and 3) for large cables, it is difficult to install the termination due to the bottle""s fixed position. An additional drawback includes losing electrical contact if vibration is present. Electrical contact can be lost in the oil-bottle method because the conductor only contacts the electrode via the steel balls. Vibration may be great enough to overcome the frictional forces holding the conductor in place.
Applicants have discovered that conventional techniques do not provide a cable termination assembly for cable quality testing that provides one termination design to fit a large range of cable constructions, that does not require different insulation shield strip-back distances, that provides a secure electrical connection, and that is environmentally friendly.
In accordance with the current invention, a cable termination assembly is provided that avoids the problems associated with prior art cable terminations as discussed herein above.
In one aspect, a method for testing an electrical power cable consistent with the invention includes providing a conductor with a first length of exposed conductor and a second length of materials layered around the conductor. Once the conductor is provided, a cable grip assembly is fastened to the first length of exposed conductor. Preferably, layers of metallic tape may be applied around the first length of the conductor. Also, a conductive sphere may be attached to the first length of the conductor and then placed in and fastened to the cable grip assembly.
After the cable grip assembly is fastened to the conductor, the cable grip assembly is connected mateably to the inside of the outer cup. Preferably, this may be accomplished by allowing the bottom outer surface of an inner cup to be attached to the inner bottom surface of an outer cup wherein the inner cup is fully contained in the outer cup. Once the two cups are attached, the cable grip assembly may be mateably connected into the top of the inner cup, wherein the inner cup completely contains the cable grip assembly. Preferably, the cable grip assembly may be connected to the inner cup by aligning keyway pins protruding from the inner cup with keyways placed in the exterior of the cable grip assembly and locking the keyway pins in the keyways. Alternatively, the cable grip assembly may be connected to the inner cup by snapping push pins protruding from the interior of the inner cup into a circumferential groove placed in the exterior of the cable grip assembly.
With the cable grip assembly connected mateably to the inside of the outer cup, the outer cup is filled with a high dielectric strength liquid covering the inner cup and the cable grip assembly. Preferably, the outer cup may comprise a tube made of an electrically non-conductive material and an end cap made of an electrically conductive material mateably connecting to one end of the tube. Once the high dielectric strength liquid has been added, a voltage is applied to the bottom of the outer cup. This voltage is transferred from the bottom of the outer cup, to the inner cup, to the cable grip assembly, and to the conductor. Preferably, the partial voltage discharge of the cable may be measured to determine the quality of the insulation.
In another aspect, a cable termination apparatus consistent with the invention includes a cable grip assembly configured to grasp and electrically connect to a first end of a conductor associated with a first end of a cable. Preferably, the conductor may be solid or stranded and may be made of copper, aluminum, or aluminum alloy. Also, the insulation layered around the conductor may be made polyethylene, crosslinked polyethylene, or ethylene-propylene rubber. In addition, layers of metallic tape may be applied around the first length of the conductor.
Preferably, the cable grip assembly may comprise a support sleeve, grip pins slideably connected to the interior of the support sleeve operable collectively to grasp the first end of the conductor, and a piston. The piston is rotatable in a first direction to cause the grip pins to grasp and electrically connect to the first end of the conductor, and rotatable in a second direction to cause the grip pins to release and electrically disconnect from the first end of the conductor. Alternatively, the cable grip assembly may comprise a support sleeve, a clamping rod traversing the interior of the support sleeve operable for rotating in a first direction and in a second direction, and opposing grip pins. The opposing grip pins are operable for moving along the clamping rod and operable collectively to grasp the first end of the conductor. The clamping rod may be rotated in a first direction to cause the grip pins to grasp and electrically connect to the first end of the conductor, and may be rotated in a second direction to cause the grip pins to release and electrically disconnect from the first end of the conductor.
Preferably, an inner cup may be mateably attached around and electrically connected to the cable grip assembly. Preferably, the cable grip assembly may be connected to the inner cup by aligning keyway pins protruding from the inner cup with keyways placed in the exterior of the cable grip assembly and locking the keyway pins in the keyways. Alternatively, the cable grip assembly may be connected to the inner cup by snapping push pins protruding from the interior of the inner cup into a circumferential groove placed in the exterior of the cable grip assembly. In addition, slots may be placed in the exterior of the cable grip assembly perpendicular to the circumferential groove and extending from the circumferential groove to the end of the cable grip assembly. The cable grip assembly may be released from the inner cup by rotating the cable grip assembly until the push pins align with the slots and by pulling the cable grip assembly from the inner cup.
And finally, an outer cup is mateably attached around the cable grip assembly. Preferably, this may be accomplished by attaching the cable grip assembly to an inner cup and then attaching an outer cup mateably around the inner cup. The outer cup contains a high dielectric fluid covering the cable grip assembly, and covering a length of the first end of the conductor. Preferably, the outer cup may comprise a tube made of an electrically non-conductive material and an end cap made of an electrically conductive material mateably connecting to one end of the tube. In addition, the cable termination apparatus may also include a voltage source configured to supply voltage to the first end of the conductor via the outer cup, the inner cup, and the cable grip assembly.
In yet another aspect, a method for testing an electrical power cable consistent with the invention includes a conductor with a first length of exposed conductor and a second length of materials layered around the conductor. Once the conductor is provided a first tube is fastened through the bottom of a second tube. Next, the cable is placed through the second tube and down through the first tube, wherein the first length of exposed conductor protruding from the first tube. Then the second tube is filled with a high dielectric strength liquid and a voltage is applied to the first length of exposed conductor. Preferably, the partial voltage discharge of the cable may be measured to determine the quality of the insulation.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are intended to provide further explanation of the invention as claimed. The following description, as well as the practice of the invention, set forth and suggest additional advantages and purposes of the invention.